Visible in the metaphysis of a normal hip is a pattern of support and traction trabeculae, which pattern corresponds with the pattern of mechanical forces. In the course of extensive research a prosthesis has been developed possessing a form such that the natural bio-geometrical forces pattern is approximated as closely as possible in the femur metaphysis. In other words the femoral prosthesis must be arranged in the femur such that during loading the centre of gravity of the femoral prosthesis lies as closely as possible to or coincides with the metaphysial centre point.
FIGS. 5, 6 and 7 in the drawing are femur sections showing different supporting possibilities of the femoral prosthesis, whereby for the sake of clarity the contact surfaces are shown enlarged and shaded.
In FIG. 5 the prosthesis 2 arranged in the femur 1 supports medially on the pubic arch ("Adamse boog") S.sub.1 and laterally on the cortex S.sub.2. In the case of unipodal loading G of the hip head 3 the centre of gravity CG will come to lie during the resolution of forces very close the pubic arch S.sub.1 and at a large interval from the metaphysial centre CM. The overpressure created at the pubic arch S.sub.1 has to be compensated by for instance a flange supporting against the greater trochanter 4.
In the situation shown in FIG. 6 the prosthesis 2 supports medially on the pubic arch S.sub.1 and laterally on the greater trochanter S.sub.3. When unipodal loading G occurs the centre of gravity CG is located in the neck 5 of the prosthesis and once again at an interval from the metaphysial centre.
FIG. 7 shows the ideal loading according to the invention, whereby the prosthesis 2 supports medially on the pubic arch S.sub.1 and laterally on the lateral cortex S.sub.2 and the greater trochanter S.sub.3. In the case of unipodal loading G the centre of gravity CG and the metaphysial centre CM now coincide. It is the loading shown in FIG. 7 which is approximated as closely as possible with the prosthesis according to the invention.
It is noted that the loadings of the femur 1 shown in FIGS. 5-7 have been confirmed in mechanical loading experiments with plastic prosthesis elements, whereby, using polarised light the forces/loading pattern was visualised.
The invention relates to a stem for a hip prosthesis and the hip prosthesis itself, which stem is provided for medial support on the pubic arch (S.sub.1) with a curved, smooth medial stem support part, is provided for lateral support on the lateral cortex (S.sub.2) with a lateral stem support part, and is provided for lateral support on the greater trochanter (S.sub.3) with a lateral comb-like stem support part located between the lateral stem support part and the head end and comprising a number of combs, the comb connecting onto the lateral side protruding laterally relative thereto, and which stem is provided on the ventral and dorsal side at the point of the metaphysial portion with at least one groove which consists of groove portions converging in the direction of the head end and enclosing an apex. Achieved through this manner of supporting is that the centre of gravity of the prosthesis coincides with or lies in the direct vicinity of the metaphysial centre, and in this way duplicates as closely as possible the natural bio-geometrical forces in the femoral metaphysis.
For the lateral supporting on the greater trochanter S.sub.3 it is favourable that the comb-shaped stem support part comprises a number of combs and that the comb connecting onto the lateral side protrudes laterally relative thereto. This comb-like structure for the stem support part prevents a distal sliding of the prosthesis when loading occurs, after ingrowth into the bone of the greater trochanter, while in addition excessive loading of the lateral cortex S.sub.2 is countered and therefore the rotation moment in the case of unipodal loading.
The stem is provided on the ventral and dorsal side at the point of the metaphysial portion with at least one groove. These grooves contribute to the anchoring of the prosthesis in the femoral metaphysis. The form of the grooves corresponds with that of the natural support and traction trabeculae. During driving of the prosthesis into the femur released trabecular structures moreover accumulate in the grooves, particularly at the point of the apex, and form ventral and dorsal strips of bone which contribute to a rapid anchoring of the prosthesis in the femur. Since the ventral and dorsal side surfaces of the stem diverge from one another towards the head end at the height of the grooves, a distal sliding of the prosthesis when loading occurs is prevented.
It is favourable if the radius of curvature of the curved medial stem support part amounts to 120-150 cm is preferably 130-140 cm and more preferably 135 cm. The prosthesis thus supports on the medial calcar and the meta-diaphysial portion of the femur, and this preferably over a length of approximately 10 cm.
For a good balance between the medial supporting on the pubic arch S.sub.1 and the lateral supporting on the cortex S.sub.2 it is favourable that in the direction towards the femur end a flat medial side portion connects onto the curved medial stem support part, which portion diverges relative to the flat lateral side facing away from it, whereby in preference the angle of divergence amounts to 1.degree.-5.degree., is preferably 2.degree.-4.degree. and more preferably 3.degree.. It is thus possible that the prosthesis supports laterally on the meta-diaphysial a transition, i.e. on the point where the cortex of the femur has its highest diameter, and the supporting takes place over a length of approximately 3 cm.
The stem preferably possesses a substantially rounded rectangular section. An unnatural loading of the femoral diaphysis is thus avoided. Were the diaphysis to come into complete contact with the prosthesis stem there would then occur a stiffening in the elasticity of the femoral diaphysis, which can have negative effects on the anchoring on the lateral cortex.
At least the metaphysial portion of the stem is preferably coated with hydroxyapatite. This hydroxyapatite coating contributes to an accelerated and improved ingrowth of the bone. Because mainly the metaphysial portion of the stem is in contact with the femur, at least this portion of the stem has to be coated with hydroxyapatite.
Finally, it is remarked that the prosthesis has a form such that in the case of a femur break or as a result of other unexpected circumstances the prosthesis is easy to remove without causing any great damage to the femur.
Mentioned and other features of the hip prosthesis and hip prosthesis stem according to the invention will be elucidated hereinafter on the basis of the description of an embodiment of the prosthesis, which is given only by way of example, while reference is made to the annexed drawing.